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Message-ID: <20260120195407.1163051-3-hpa@zytor.com>
Date: Tue, 20 Jan 2026 11:53:54 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Uros Bizjak <ubizjak@...il.com>, Petr Mladek <pmladek@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <kees@...nel.org>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Nathan Chancellor <nathan@...nel.org>,
Kiryl Shutsemau <kas@...nel.org>,
Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
linux-coco@...ts.linux.dev, x86@...nel.org
Subject: [PATCH v1 02/14] x86/realmode: make %gs == 0 an invariant
When accessing data that is not "near", either only one segment is
used or one segment is always zero. Leave %gs == 0 at all times
throughout the C code; this reduces the number of segment loads
needed.
Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
---
arch/x86/boot/a20.c | 11 +++++------
arch/x86/boot/header.S | 3 +++
arch/x86/boot/regs.c | 3 ++-
arch/x86/boot/video-bios.c | 5 ++---
arch/x86/boot/video-mode.c | 5 ++---
arch/x86/boot/video.c | 7 +++----
arch/x86/realmode/rm/wakeup_asm.S | 10 ++++++----
7 files changed, 23 insertions(+), 21 deletions(-)
diff --git a/arch/x86/boot/a20.c b/arch/x86/boot/a20.c
index bda042933a05..3ab6cd8eaa31 100644
--- a/arch/x86/boot/a20.c
+++ b/arch/x86/boot/a20.c
@@ -56,20 +56,19 @@ static int a20_test(int loops)
int ok = 0;
int saved, ctr;
- set_fs(0x0000);
- set_gs(0xffff);
+ set_fs(0xffff);
- saved = ctr = rdfs32(A20_TEST_ADDR);
+ saved = ctr = rdgs32(A20_TEST_ADDR);
while (loops--) {
- wrfs32(++ctr, A20_TEST_ADDR);
+ wrgs32(++ctr, A20_TEST_ADDR);
io_delay(); /* Serialize and make delay constant */
- ok = rdgs32(A20_TEST_ADDR+0x10) ^ ctr;
+ ok = rdfs32(A20_TEST_ADDR+0x10) ^ ctr;
if (ok)
break;
}
- wrfs32(saved, A20_TEST_ADDR);
+ wrgs32(saved, A20_TEST_ADDR);
return ok;
}
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 9bea5a1e2c52..bda20395658f 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -596,6 +596,9 @@ start_of_setup:
shrw $2, %cx
rep stosl
+# The C code uses %gs == 0 as invariant
+ movw %ax, %gs
+
# Jump to C code (should not return)
calll main
diff --git a/arch/x86/boot/regs.c b/arch/x86/boot/regs.c
index 55de6b3092b8..54d6dfd129c5 100644
--- a/arch/x86/boot/regs.c
+++ b/arch/x86/boot/regs.c
@@ -22,6 +22,7 @@ void initregs(struct biosregs *reg)
reg->eflags |= X86_EFLAGS_CF;
reg->ds = ds();
reg->es = ds();
+ /* The input values of %cs and %ss are ignored by intcall() */
reg->fs = fs();
- reg->gs = gs();
+ /* %gs == 0 */
}
diff --git a/arch/x86/boot/video-bios.c b/arch/x86/boot/video-bios.c
index 6eb8c06bc287..e8be64424a40 100644
--- a/arch/x86/boot/video-bios.c
+++ b/arch/x86/boot/video-bios.c
@@ -73,7 +73,6 @@ static int bios_probe(void)
if (adapter != ADAPTER_EGA && adapter != ADAPTER_VGA)
return 0;
- set_fs(0);
crtc = vga_crtc();
video_bios.modes = GET_HEAP(struct mode_info, 0);
@@ -105,8 +104,8 @@ static int bios_probe(void)
mi = GET_HEAP(struct mode_info, 1);
mi->mode = VIDEO_FIRST_BIOS+mode;
mi->depth = 0; /* text */
- mi->x = rdfs16(0x44a);
- mi->y = rdfs8(0x484)+1;
+ mi->x = rdgs16(0x44a);
+ mi->y = rdgs8(0x484)+1;
nmodes++;
}
diff --git a/arch/x86/boot/video-mode.c b/arch/x86/boot/video-mode.c
index 9ada55dc1ab7..e5b9bc96bd42 100644
--- a/arch/x86/boot/video-mode.c
+++ b/arch/x86/boot/video-mode.c
@@ -119,9 +119,8 @@ static void vga_recalc_vertical(void)
u16 crtc;
u8 pt, ov;
- set_fs(0);
- font_size = rdfs8(0x485); /* BIOS: font size (pixels) */
- rows = force_y ? force_y : rdfs8(0x484)+1; /* Text rows */
+ font_size = rdgs8(0x485); /* BIOS: font size (pixels) */
+ rows = force_y ? force_y : rdgs8(0x484)+1; /* Text rows */
rows *= font_size; /* Visible scan lines */
rows--; /* ... minus one */
diff --git a/arch/x86/boot/video.c b/arch/x86/boot/video.c
index 0641c8c46aee..09b810faa5c0 100644
--- a/arch/x86/boot/video.c
+++ b/arch/x86/boot/video.c
@@ -79,12 +79,11 @@ static void store_mode_params(void)
video_segment = 0xb800;
}
- set_fs(0);
- font_size = rdfs16(0x485); /* Font size, BIOS area */
+ font_size = rdgs16(0x485); /* Font size, BIOS area */
boot_params.screen_info.orig_video_points = font_size;
- x = rdfs16(0x44a);
- y = (adapter == ADAPTER_CGA) ? 25 : rdfs8(0x484)+1;
+ x = rdgs16(0x44a);
+ y = (adapter == ADAPTER_CGA) ? 25 : rdgs8(0x484)+1;
if (force_x)
x = force_x;
diff --git a/arch/x86/realmode/rm/wakeup_asm.S b/arch/x86/realmode/rm/wakeup_asm.S
index 02d0ba16ae33..a8a8580158d7 100644
--- a/arch/x86/realmode/rm/wakeup_asm.S
+++ b/arch/x86/realmode/rm/wakeup_asm.S
@@ -70,15 +70,17 @@ SYM_CODE_START(wakeup_start)
movl $rm_stack_end, %esp
movw %ax, %ds
movw %ax, %es
- movw %ax, %fs
- movw %ax, %gs
- lidtl .Lwakeup_idt
+ xorl %eax, %eax
+ movw %ax, %fs
+ movw %ax, %gs /* The real mode code requires %gs == 0 */
/* Clear the EFLAGS */
- pushl $0
+ pushl %eax
popfl
+ lidtl .Lwakeup_idt
+
/* Check header signature... */
movl signature, %eax
cmpl $WAKEUP_HEADER_SIGNATURE, %eax
--
2.52.0
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